In recent years, a spindle motor to be incorporated into a disc drive device, such as a hard disc drive device (hereinafter, referred to as HDD) employs a dynamic pressure fluid bearing (hereinafter, referred to as fluid bearing) which can realize low non-repetitive run out (NRRO) and low noise by non-contact rotation.
The fluid bearing has a structure that a fluid (hereinafter, referred to as a lubricant) is filled between a stationary portion and a rotating portion. In order to achieve an operation with a high precision, it is important to prevent leakage and evaporation of the lubricant.
In general, in such a fluid bearing, a seal mechanism is provided on an opening end in order to prevent leakage of the lubricant from the opening end. One of the seal mechanisms is to provide a tapered structure in an upper part of the rotating portion in an axial direction to form a gap between the fixed portion and the rotation portion into a tapered shape which becomes wider as it extends from the inner side of the bearing toward outside. This is effective not only in preventing the leakage of the lubricant by utilizing a surface tension of the lubricant, but also in facilitating filling the gap with the lubricant by utilizing a capillary phenomenon when the lubricant is injected.
Meanwhile, if the fluid bearing is left under a high temperature, for example, the lubricant may be leaked out. In such a case, if the temperature of the lubricant rises, and a surface area of the lubricant becomes large as it leaked out, the rate of evaporation of the lubricant increases, and the lubricant evaporates rapidly. Therefore, in general, a lube repellant is applied to a portion near the bearing opening end in order to prevent leakage of the lubricant which may result in speeding up evaporation of the lubricant.
In order to more effectively achieve the effects of the seal mechanism and the lubricant (hereinafter, referred to as sealing function), various types of fluid bearings have been proposed.
Examples include: a structure in which a wide groove for applying a lube repellant is provided on a lower surface of a hub (see Japanese Laid-Open Publication No. 2001-248645); a structure in which a lube repellant is applied to a sleeve and an upper end portion of a shaft (see Japanese Laid-Open Publication Nos. 2001-304263 and 2004-263814); a structure in which a concave portion or a convex portion is provided on a tapered structure to define a position to apply a lube repellant (see Japanese Laid-Open Publication No. 2003-65336); and the like.